This invention relates to apparatus for signalling within a borehole while drilling, and is more particularly concerned with a down-hole signal transmitter for a mud-pulse telemetry system.
Various types of measurements-while-drilling (MWD) systems have been proposed for taking measurements within a borehole while drilling is in progress and for transmitting the measurement data to the surface. However to date only one type of system has enjoyed commercial success, that is the so-called mud-pulse telemetry system. In that system the mud stream, which passes down the drill string to the drill bit and then back up the annular space between the drill string and the bore wall with the object of lubricating the drill string and carrying away the drilling products, is used to transmit the measurement data from a down-hole measuring instrument to a receiver and data processor at the surface. This is achieved by modulating the mud pressure in the vicinity of the measuring instrument under control of the electrical output signal from the measuring instrument, and sensing the resultant mud-pulses at the surface by means of a pressure transducer.
Current mud-pulse telemetry systems utilize a down-hole signal transmitter which is built into the drill collar. These systems therefore suffer from the disadvantage that, in the event of instrumentation failure in the transmitter, the complete drill string must be withdrawn to enable the faulty part to be replaced. Moreover the combined transmitter/drill collar is very costly to produce. It is an object of the invention to provide a generally improved down-hole signal transmitter for a mud-pulse telemetry system.